Method for manufacturing hot metal desulfurizing agent and apparatus for same

ABSTRACT

The present invention provides a method of desulfurizing hot metal, which method utilizes desulfurization slag resulting from a KR hot metal desulfurizing treatment as a desulfurizing agent for hot metal again to reduce hot metal desulfurization costs and the amount of slag generated, thereby solving environmental problems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of PCT Application No.PCT/JP01/05065, filed Jun. 14, 2001, which was not published under PCTArticle 21(2) in English.

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2000-178321, filed Jun. 14,2000, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing adesulfurizing agent for hot metal and a desulfurizing agent, and inparticular, to a method of manufacturing a desulfurizing agent for hotmetal, which method effectively reuses desulfurization slag (KR slag)resulting from a mechanical agitation type hot metal desulfurizingtreatment step, an apparatus for use in this method, a desulfurizingagent (flux) for use in this method and apparatus, and a method ofdesulfurizing hot metal using this desulfurizing agent.

2. Description of the Related Art

Hot metal output from a blast furnace contains a high concentration ofsulfur (S), which normally affects the quality of steel. However, sincea converter step is intended to oxidize and remove impurities, moltensteel is not expected to be desulfurized except for part thereof whichis vaporized and desulfurized. Thus, depending on the desired quality,various hot metal pretreatments are executed between the blast furnacestep and the converter step or molten steel is desulfurized after theconverter step. FIG. 23 shows an example of a hot metal pretreatment. Inthe illustrated example, hot metal from the blast furnace issequentially subjected to a desiliconizing treatment, a desulfurizingtreatment, and a dephosphorizing treatment. The hot metal is then placedin the converter, where it is subjected to a decarburizing treatment.

For desulfurization, a lime-based desulfurizing agent is often used. Adesulfurizing reaction in this case proceeds according to the reactionformula shown below.CaO+S→CaS+O

With only CaO, such a desulfurizing reaction involves a high meltingpoint. Thus, typically, fluorite, alumina-based flux, or the like isindustrially used to facilitate slagging. However, these slagging agentsare generally expensive, so that an increase in the compounding rate ofsuch a slagging agent leads to an increase in the costs of thedesulfurizing agent. Furthermore, an increase in the compounding rate ofthe slagging agent may reduce the concentration of lime in thedesulfurizing agent to degrade reaction effects.

Further, slag resulting from a pyrometalurgy step in a blast furnace ora converter is reused for blast furnace cement, concrete material,fertilizers, or road material after having contained metal removed.However, desulfurization slag is characterized by containing a largeamount of CaO and being likely to be weathered. Accordingly, there is noother way but to use the desulfurizing slag for cement material byexecuting a pretreatment that requires much time and labor. Furthermore,at present, this treatment involves high cost.

Jpn. Pat. Appln. KOKAI Publication No. 4-120209 describes a technique ofutilizing converter slag as a slagging agent. This application statesthat the grain size of the blast furnace slag is set at 3 to 50 mm andthat this range of grain size serves to produce a sufficientdephosphorizing effect. However, this application is not mainly intendedfor desulfurization, and has no references to desulfurization.

Further, Jpn. Pat. Appln. KOKAI Publication No. 10-30115 discloses atechnique of cooling and crushing converter slag, separating andrecovering the iron component from the slag, and compounding lime andfluorite into the slag so that it can be used as a desulfurizing agent.However, this application has no references to the recycling ofdesulfurization slag as in the above technique.

As an example of the recycling of desulfurization slag, there is areport on a process of reusing injection desulfurization slag containinga large amount of unreacted lime, in a mechanical agitation type hotmetal desulfurizing treatment which allows lime to be used efficiently(Sumitomo Metals Vol. 45-3 (1993) p. 52 to 58). However, with thetreatment (hereinafter referred to as the “prior art of desulfurizationslag recycling”) reported in this document, there is a limit to theimprovement of efficiency of utilization of lime as described later.Further, since this technique reuses the slag in the process usingdifferent addition and agitation methods, it is not applicable if aplurality of processes are not available.

A mechanical agitation type hot metal desulfurization apparatusdesulfurizes hot metal by immersing and rotating an impeller in hotmetal, adding a desulfurizing agent (normally lime) to the hot metalfrom above, and rotating the impeller to agitate the hot metal. Aprocess commonly known as the “KR method” is a hot metal desulfurizingtreatment using this apparatus. FIG. 24 shows an example of adesulfurizing facility.

As described above, at present, desulfurization slag resulting from ahot metal desulfurizing treatment is not effectively recycled. Further,for desulfurization of hot metal, there are many points to be improved.

BRIEF SUMMARY OF THE INVENTION

The inventors examined the efficiency of utilization of a desulfurizingagent used in the KR desulfurizing step. FIG. 18 shows the ratio of limeeffectively used to introduced lime wherein the case in whichdesulfurization slag resulting from a mechanical agitation type hotmetal desulfurizing treatment method is used as a desulfurizing agentfor the mechanical agitation type hot metal desulfurizing treatmentmethod is compared with the case in which desulfurization slag resultingfrom an injection method is used as a desulfurizing agent for themechanical agitation type hot metal desulfurizing treatment method. Asindicated in FIG. 18, the inventors have found that the efficiency ofutilization of the desulfurizing agent during a single KR desulfurizingstep corresponds to about 7% of the total desulfurizing agent, with theremaining 93% unreacted. Accordingly, on the basis of this knowledge,the inventors expected that the desulfurizing agent that has been usedin the KR desulfurizing step can be reused as an inexpensive source oflime for the desulfurizing agent because it still contains about 93% oflime that contributes to desulfurization in the subsequent treatment.

The present invention is provided on the basis of this knowledge. It isan object of the present invention to provide a method of desulfurizinghot metal, which method effectively reuses desulfurization slagresulting from a hot metal desulfurizing treatment in order to reducethe costs of hot metal desulfurization and the amount of slag generated.

It is another object of the present invention to inexpensively carry outa hot metal desulfurizing treatment with a reduced amount of slaggenerated and to provide a desulfurizing agent for use in thistreatment.

It is yet another object of the present invention to provide a method oftransporting desulfurization slag and an apparatus used to manufacturedesulfurization slag.

The present invention is provided to accomplish these objects.

A method of manufacturing a hot metal desulfurizing agent according tothe present invention comprises the following steps:

1. A method of manufacturing a hot metal desulfurizing agent, the methodcomprising executing a treatment for creating a new surface indesulfurization slag resulting from a mechanical agitation type hotmetal desulfurizing treatment.

2. A method of manufacturing a hot metal desulfurizing agent for use ina mechanical agitation type hot metal desulfurizing treatment, themethod comprising executing a treatment for creating a new surface indesulfurization slag resulting from the mechanical agitation type hotmetal desulfurizing treatment.

3. A method comprising a step of providing desulfurization slagresulting from the mechanical agitation type hot metal desulfurizingtreatment and a step of executing the treatment for creating a newsurface in the provided desulfurization slag.

4. A method wherein the step of executing the treatment for creating anew surface includes crushing a desulfurization slag grain and/orseparating an aggregate of a plurality of desulfurization slag grainsinto desulfurization slag grains.

5. A method wherein the step of executing the treatment for creating anew surface includes crushing the desulfurization slag grain and/orseparating the aggregate of a plurality of desulfurization slag grainsinto the desulfurization slag grains by air-cooling the desulfurizationslag and/or applying mechanical energy to the desulfurization slag.

6. A method wherein the desulfurization slag is cooled using one or twomethods selected from a group consisting of air cooling and watercooling.

7. A method wherein the air cooling is carried out using one or twomethods selected from a group consisting of natural cooling and forcedair cooling.

8. A method wherein the step of executing the treatment for creating anew surface by water cooling comprises a step of watering thedesulfurization slag, and this watering step controls the amount ofwater provided so as to maintain the temperature of the desulfurizationslag at 100° C. or higher at the end of the watering, so that only bymeans of cooling based on the watering, the aggregate of desulfurizationslag can be separated into desulfurization slag grains and/or thedesulfurization slag grains can be crushed.

9. A method wherein the step of executing the treatment for creating anew surface comprises a step of water-cooling the desulfurization slagand a step of drying the recovered desulfurizing agent resulting fromthe water cooling.

10. A method wherein the step of executing the treatment for creating anew surface comprises a step of cooling the desulfurization slag and astep of adjusting the grain sizes of the desulfurization slag and therecovered desulfurizing agent.

11. A method wherein the step of adjusting the grain size of therecovered desulfurizing agent using a sieve is executed at a temperatureof 600° C. or higher.

12. A method wherein the step of executing the treatment for creating anew surface comprises a step of executing at least one or two treatmentsselected from a group consisting of a treatment for magneticallyseparating and removing metal from the desulfurization slag or therecovered desulfurizing agent, a treatment for removing large massesfrom the desulfurization slag or the recovered desulfurizing agent toset the grain size at 100 mm or smaller, and a treatment for setting thetemperature of the desulfurization slag or the recovered desulfurizingagent at 200° C. or lower.

13. A method wherein the step of executing the treatment for creating anew surface comprises a step of setting the grain size of thedesulfurization slag at 100 mm or smaller and the temperature thereof at200° C. or lower.

14. A method of transporting a recovered desulfurizing agent, the methodcomprising a step of loading a recovered desulfurizing agent resultingfrom a mechanical agitation type hot metal desulfurizing treatment ontoa transport vehicle using a pair of movable basket sections that can beopened and closed and a step of using the transport vehicle to transportthe recovered desulfurizing agent to a desulfurizing treatment facility.

15. A method comprising a step of screening the cooled and crushedrecovered desulfurizing agent to remove large masses before orsimultaneously with the step of using the transport vehicle to transportthe recovered desulfurizing agent.

16. A method of transporting a recovered desulfurizing agent, the methodcomprising a step of loading a recovered desulfurizing agent resultingfrom a mechanical agitation type hot metal desulfurizing treatment ontoa transport vehicle having a capability of sucking the recovereddesulfurizing agent and a step of using the transport vehicle totransport the recovered desulfurizing agent to a desulfurizing treatmentfacility.

17. A method wherein the step of loading the recovered desulfurizingagent is executed while adjusting a height from which the recovereddesulfurizing agent is dropped onto the transport vehicle, to 1.5 m orless.

18. An apparatus which screens a recovered desulfurizing agent, theapparatus comprising an apparatus main body having a sieve mesh thatscreens a crushed recovered desulfurizing agent and an air sucking hoseattached to the apparatus main body to facilitate suction of therecovered desulfurizing agent into the apparatus main body.

19. An apparatus which screens a recovered desulfurizing agent, theapparatus comprising a member having a diagonally arranged sieve mesh, adiagonal plate diagonally arranged below the member and on which a minussieve slides down, and a sliding way on which the minus sieve from thediagonal plate falls and then slides down, and wherein the apparatus isarranged so that the spacing between the member having the sieve meshand the diagonal plate and the vertical height of drop of a linkagebetween the diagonal plate and the sliding way are each 500 mm or lessand the height of drop from the sliding way to a ground surface is 500mm or less.

20. A hot metal desulfurizing agent comprising a recovered desulfurizingagent resulting from a mechanical agitation type hot metal desulfurizingtreatment.

21. A hot metal desulfurizing agent comprising the recovereddesulfurizing agent resulting from the mechanical agitation type hotmetal desulfurizing treatment, the hot metal desulfurizing agent beingused for the mechanical agitation type hot metal desulfurizingtreatment.

22. A hot metal desulfurizing agent comprising the recovereddesulfurizing agent resulting from the mechanical agitation type hotmetal desulfurizing treatment and having a new surface created therein.

23. A hot metal desulfurizing agent comprising the recovereddesulfurizing agent which results from the mechanical agitation type hotmetal desulfurizing treatment and in which part or all of an aggregateof recovered desulfurizing agent grains is separated into pieces.

24. A hot metal desulfurizing agent having a maximum grain size of 100mm or smaller.

25. A hot metal desulfurizing agent comprising the recovereddesulfurizing agent resulting from the mechanical agitation type hotmetal desulfurizing treatment, the hot metal desulfurizing agent furthercontaining one or two sources selected from a group consisting of a limesource and a carbon source.

26. A hot metal desulfurizing agent wherein the one or two sourcesselected from the group consisting of the lime source and the carbonsource are mixed with the recovered desulfurizing agent resulting fromthe mechanical agitation type hot metal desulfurizing treatment, andthis mixture is added to hot metal.

27. A hot metal desulfurizing agent wherein the one or two sourcesselected from the group consisting of the lime source and the carbonsource are separated from the recovered desulfurizing agent resultingfrom the mechanical agitation type hot metal desulfurizing treatment,and the one or more sources and the recovered desulfurizing agent areseparately added to hot metal.

28. A hot metal desulfurizing agent wherein the lime source is at leastone or two selected from a group consisting of lime, calcium carbonate,and calcium hydroxide.

29. A hot metal desulfurizing agent wherein the total amount of calciumcarbonate and calcium hydroxide, which are included in the group of limesources, corresponds to 40 wt % or less of the whole hot metaldesulfurizing agent.

30. A hot metal desulfurizing agent wherein the amount of carbon sourcecorresponds to 30 wt % or less of the whole hot metal desulfurizingagent.

31. A hot metal desulfurizing agent wherein the carbon source is powdersof grain size 1 mm or smaller.

32. A hot metal desulfurizing agent wherein the carbon source is atleast one or two selected from a group consisting of coal, coke, andpitch.

33. A method of manufacturing low-sulfur hot metal, the methodcomprising desulfurizing hot metal by adding, to the hot metal, a hotmetal desulfurizing agent comprising a recovered desulfurizing agentresulting from a mechanical agitation type hot metal desulfurizingtreatment.

34. A method of manufacturing low-sulfur hot metal, the methodcomprising desulfurizing hot metal using a mechanical agitation type hotmetal desulfurizing treatment, by adding, to the hot metal, a hot metaldesulfurizing agent comprising a recovered desulfurizing agent resultingfrom the mechanical agitation type hot metal desulfurizing treatment.

35. A method of manufacturing low-sulfur hot metal, the methodcomprising desulfurizing the hot metal by adding, to the hot metal, thedesulfurizing agent comprising the recovered desulfurizing agentresulting from the mechanical agitation type hot metal desulfurizingtreatment and having a new surface created therein.

36. A method of manufacturing low-sulfur hot metal, the methodcomprising desulfurizing the hot metal by adding, to the hot metal, thedesulfurizing agent comprising the recovered desulfurizing agent whichresults from the mechanical agitation type hot metal desulfurizingtreatment and in which part or all of an aggregate of the recovereddesulfurizing agent is separated into pieces.

37. A method comprising desulfurizing the hot metal by adding, to hotmetal, the recovered desulfurizing agent resulting from the mechanicalagitation type hot metal desulfurizing treatment and one or more sourcesselected from a group consisting of a lime source and a carbon source.

38. A method comprising desulfurizing the hot metal by mixing therecovered desulfurizing agent resulting from the mechanical agitationtype hot metal desulfurizing treatment with one or more sources selectedfrom a group consisting of a lime source and a carbon source, and addingthis mixture to hot metal.

39. A method comprising desulfurizing the hot metal by separatelyadding, to the hot metal, the recovered desulfurizing agent resultingfrom the mechanical agitation type hot metal desulfurizing treatment andone or more sources selected from a group consisting of a lime sourceand a carbon source.

40. A method wherein when the lime source is added, the mixing ratio isadjusted so as to obtain a predetermined amount of pure CaO component.

41. A method comprising a step of calculating the bulk density of therecovered desulfurizing agent resulting from the mechanical agitationtype hot metal desulfurizing treatment, a step of calculating the amountof pure CaO component in the recovered desulfurizing agent from thecalculated bulk density, and a step of adjusting the addition ratio ofthe crushed recovered desulfurizing agent to the lime source on thebasis of the calculated amount of pure CaO component in the recovereddesulfurizing agent.

42. A method comprising a step of adjusting the grain size of the carbonsource to 1 mm or smaller when the carbon source is added.

(Definition)

“Desulfurizing agent” refers to common fluxes used for desulfurization.It includes slag recovered after a treatment for creating a new surface.

“Recovered desulfurizing agent” refers specifically to a desulfurizingagent comprising slag recovered after the treatment for creating a newsurface, wherein the slag may contain metal.

“Desulfurization slag” refers to desulfurization slag containing all ofa CaO component, other slag components, and a metal component and whichhas not been subjected to the treatment for creating a new surface.

“Lime” refers to common CaO components.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a graph illustrating the relationship between the amount oflime added per unit amount of hot metal and desulfurization rate, forrecovered desulfurizing agents resulting from the first and secondrecycling operations, respectively, according to the present inventionas well as a conventional lime desulfurizing agent for comparison.

FIG. 2 is a graph illustrating the relationship between the number oftimes of recycling processes executed on desulfurization slag and theamount of desulfurizing agent used per unit amount of hot metal whereinthe slag is recycled a number of times according to the presentinvention.

FIG. 3 is a graph illustrating the relationship between the amount ofdesulfurizing agent added per unit amount of hot metal and thedesulfurization rate, for the desulfurizing agent according to thepresent invention and the conventional desulfurizing agent forcomparison.

FIG. 4 is a graph illustrating the relationship between the amount oflime component added per unit amount of hot metal and thedesulfurization rate, for the desulfurizing agent according to thepresent invention and the conventional desulfurizing agent forcomparison.

FIG. 5 is a graph illustrating the relationship between the amount oflime added per unit amount of hot metal and the desulfurization rate,for the recovered desulfurizing agent treated according to the presentmethod and the conventional desulfurizing agent for comparison.

FIG. 6 is a graph illustrating the relationship between the temperatureof slag measured at the end of watering and the time required to coolthe slag down to room temperature, wherein a treatment is executed usinga desulfurization slag treatment method according to the presentinvention.

FIG. 7 is a graph illustrating the relationship between the temperatureof the slag measured at the end of watering and the amount of Ca(OH)₂generated in the recovered desulfurizing agent, wherein a treatment isexecuted using the desulfurization slag treatment method according tothe present invention.

FIG. 8 is a graph illustrating the relationship between the timerequired to cool slag with different thicknesses and the temperature ofthe slag, wherein a treatment is executed using the desulfurization slagtreatment method according to the present invention.

FIG. 9 is a graph showing the relationship between the amount of limeadded per unit amount of hot metal and desulfurization rate, for therecovered desulfurizing agent treated according to the present inventionand the conventional desulfurizing agent for comparison.

FIG. 10 is a graph showing the relationship between the amount of limeadded per unit amount of hot metal and desulfurization rate, for therecovered desulfurizing agent recycled according to the presentinvention and the conventional desulfurizing agent for comparison.

FIG. 11 is a view showing an example of a screening jig according to thepresent invention.

FIG. 12 is a view showing a screening facility according to the presentinvention.

FIG. 13A is a view useful in describing an essential part of a recovereddesulfurizing agent loading apparatus, and FIG. 13B is a schematic viewshowing the entire recovered desulfurizing agent loading apparatus.

FIG. 14 is a graph showing the amount of pure CaO component in thedesulfurizing agent and the amount of material desulfurized (the amountof sulfur before treatment (S)−the amount of sulfur after treatment(S)).

FIG. 15 is a graph showing the bulk density and CaO mass % ofdesulfurization slag.

FIG. 16 is a graph showing the relationship between the amount of limeadded per unit amount of hot metal and the desulfurization rate, fordesulfurizing agents obtained by mixing the recovered desulfurizingagent recovered according to the present invention with lime and aconventional desulfurizing agent as a comparative example.

FIG. 17A is a view showing a slag treatment pattern performed by a planttest.

FIG. 17B is a view showing an example of the slag treatment pattern inFIG. 17A together with a conventional slag treatment pattern.

FIG. 18 is a graph showing the ratio of lime effectively used fordesulfurization to introduced lime, by comparing the case in whichdesulfurizing agent resulting from a mechanical agitation type hot metaldesulfurizing treatment method is used as a desulfurizing agent for themechanical agitation type hot metal desulfurizing treatment with thecase in which desulfurization slag resulting from an injection method isused as a desulfurizing agent for the mechanical agitation type hotmetal desulfurizing treatment method.

FIG. 19 is a graph showing the relationship between the amount of limecomponent added per unit amount of hot metal and the desulfurizationrate at each desulfurization level.

FIG. 20 is a graph showing the relationship between the amount of limecomponent added per unit amount of hot metal and the desulfurizationrate at each desulfurization level.

FIG. 21 is a photograph showing how an aggregate of desulfurization slagresulting from the mechanical agitation type hot metal desulfurizingtreatment was observed using an SEM, and a view showing the results ofline analysis of an S element in the aggregate.

FIG. 22 is a view schematically showing differences between agitatingtype hot metal desulfurization slag and hot metal desulfurization slagobtained by the injection method.

FIG. 23 is a view showing an example of a hot metal pretreatment.

FIG. 24 is a view showing an example of a desulfurizing facility, shownin FIG. 23.

DETAILED DESCRIPTION OF THE INVENTION

(Recycling of Desulfurization Slag)

According to a hot metal desulfurizing method of the present invention,desulfurization slag resulting from mechanical agitation type hot metaldesulfurizing treatment (hereinafter referred to as the “KR method”) isreused as a desulfurizing agent for another hot metal desulfurizingtreatment (for example, desulfurization slag resulting from the KRmethod is reused as a desulfurizing agent for the KR method, ordesulfurization slag resulting from the KR method is reused as adesulfurizing agent for an injection method). The hot metaldesulfurizing treatment for which desulfurization slag is reused is notlimited-but refers to a commonly executed one. According to the presentinvention, desulfurization slag can also be reused for the same processthat results in the desulfurization slag. Accordingly, the method of thepresent invention is effective even in steel manufacturing facilitiesprovided with only hot metal desulfurizing treatment processes that usethe same addition method and agitation method. The present inventionallows desulfurization slag to be particularly efficiently andeffectively recycled if the recycling operation is applied to the KRmethod.

That is, with the injection method, fine lime powders are added to abath at a deep position thereof, and thus react while floating in thebath. Consequently, the fine powders are expected to react for only ashort time, and a desulfurization product is formed in front layers ofthe fine powders to only a small thickness. After the fine powders havecome floating to the bath surface, they are not expected be agitatedagain so as to be caught in waves of hot metal. Thus, a reaction surfacearea does not increase, and there are substantially no possibilities ofdesulfurizing reaction. Further, at this stage, aggregation starts, sothat desulfurization products on the surfaces of the individual finepowders are aggregated.

On the other hand, with the KR method, lime powders are added to thebath surface. Accordingly, the lime powders are caught in waves of hotmetal in so as to go down from the surface to interior of the bath.Further, at the beginning of the addition, desulfurizing agent powdersstart to be aggregated near the surface. As a result, the lime powersbecome “undissolved masses” with substantially unreacted lime containedtherein. Even when aggregation starts, surface areas of the powderswhich come into contact with metal react, and desulfurization productsare formed thereon. This reaction continues over the treatment time, andlong-time reaction is thus possible. With the above reaction mechanism,after desulfurization, the surfaces of the aggregated coarse grains arecovered with desulfurization products to a fixed thickness. There areonly a small amount of desulfurization products inside the grains. Thatis, unreacted lime, which is similar to fresh lime, is present insidethe grains. The inventors used an SEM to observe an aggregate ofdesulfurization slag resulting from the mechanical agitation type hotmetal desulfurizing treatment, took photographs thereof, andline-analyzed an S element therein. As a result, the inventors confirmedthat the above knowledge is correct, as shown in FIG. 21.

Thus, desulfurization slag resulting from the KR method contains coarsegrains and thus requires only a simple pretreatment for recycling,thereby reducing recycling treatment costs. Further, in most cases,unreacted lime grains are aggregated and can thus be recycled withoutusing any additional special methods of crushing material or adjustinggrain size.

On the other hand, desulfurization slag resulting from the injectionmethod, the individual fine grains are enclosed by desulfurizationproducts. Accordingly, a special crushing operation or the like isrequired to make the fine grains much finer. This increases the numberof steps and the treatment time.

As described above, the KR method accomplishes a high reactionefficiency during recycling. The surface of an unreacted lime componentcan be used directly for reaction for recycling, and is expected toachieve a reaction efficiency equivalent to that attained using freshlime. If KR slag is recycled, it can be desulfurized with the amount oflime component added per unit amount of hot metal, unreacted. Incontrast, with injection slag, a complicated pretreatment is required,or the use of a simple pretreatment requires at least double the amountof lime component.

Further, the use of KR slag reduces the treatment time (down to the sameamount as that required for a fresh agent) and effectively reduces theamount of slag generated. The KR slag serves to reduce the amount ofslag generated, for example, by 50% after one recycling process and toallow recycling to be carried out a number of times.

On the other hand, with the injection method, desulfurization productson the surfaces of individual fine grains are aggregated. Thus, forrecycling, the individual grains (primary grains) must be crushed. Afterrecycling, the grains are very fine (their grain size is half or smallerof that of the primary grain size). Thus, during recycling based on theKR method, a large number of grains are likely to scatter. Consequently,additional lime is required to make up for the loss of the scatteredlime component. For example, if only a conventional desulfurizing agentresulting from the first recycling operation is used, about 7 kg/t ofdesulfurizing agent (about 90% of lime component: 6.3 kg/T ofsubstantial lime component) is required. In contrast, if a recovereddesulfurizing agent is used, 10 kg/t of desulfurizing agent (about 66%of lime component: 6.6 kg/T of substantial lime component)+3 kg/t ofconventional desulfurizing agent resulting from the first recyclingoperation (about 90% of lime component: 2.7 kg/T of substantial limecomponent) is required. That is, 9.3 kg/T of lime component is presentin the recovered desulfurizing agent, but because of insufficientformation of effective new surfaces or a loss resulting from scattering,about 3 kg/T of desulfurizing agent may not effectively contribute todesulfurization. That is, an effective lime component takes up onlyabout 55% of the recovered desulfurizing agent, so that a supplementarydesulfurizing agent must be added, as described in the previouslydescribed document (Sumitomo Metals Vol. 45-3 (1993) p. 52 to 58).

With the KR method, as described above, the surface of coarse grains arecovered with desulfurization products to some degree, with only a smallamount of desulfurization products present inside the grains. That is,unreacted lime similar to fresh lime is present. Thus, desulfurizationis possible if weathering products are positively generated forrecycling and even if the grains after recycling are as large as orlarger than the primary grains. Furthermore, during recycling based onthe KR method, the amount of grains scattered is equal to or smallerthan that observed when lime is used. For example, if a recovereddesulfurizing agent is used, the amount of recovered desulfurizing agentused is 14 kg/t (about 50% of lime), and the amount of effective limecomponent in the added recovered desulfurizing agent is similar to thatobserved if fresh lime is added.

FIG. 18 shows a summary of the above relationships. If desulfurizationslag resulting from the KR method, an example of the present invention,is reused as a desulfurizing agent for the KR method, about 7% of thelime component added for one treatment is used for desulfurization.Effective utilization rate increases consistently with the number oftimes that the desulfurizing agent is used. In contrast, ifdesulfurization slag resulting from the injection method, a conventionaltechnique for desulfurization slag recycling, is reused as adesulfurizing agent for the KR method, the total utilization efficiencyonly slightly exceeds the utilization efficiency accomplished by one KRprocess.

Further, to help make the reader understand the above description, thespecification is accompanied by FIG. 22 schematically showingdifferences between hot metal desulfurization slag resulting from themechanical agitation type method and hot metal desulfurization slagresulting from the injection method. FIG. 22 shows how an aggregate ofdesulfurization slag grains resulting from the mechanical agitation typehot metal desulfurizing treatment is separated into pieces to create newsurfaces and how each grain of aggregate of desulfurization slag grainsresulting from the injection method is crushed into pieces. In thefigure, the desulfurization slag grains resulting from the injectionmethod have substantially the same grain size as the desulfurizationslag grains resulting from the mechanical agitation type hot metaldesulfurizing treatment. However, the desulfurization slag grainsresulting from the injection method are actually finer.

(Specific Treatment for Desulfurization Slag)

FIG. 17A shows a slag treatment pattern performed by a plant test, andFIG. 17B shows an example of the slag treatment pattern in FIG. 17Atogether with a conventional slag treatment pattern.

According to the present invention, an arbitrary method is used tocreate new surfaces in desulfurization slag resulting from adesulfurizing step, and the slag obtained is reused as a desulfurizingagent. In this case, an unreacted lime component must be exposed as adesulfurization reacting surface so that the slag can be used as adesulfurizing agent for the next process. The method used to achievethis is not limited. If a natural cooling process or a watering andcooling process is executed to create new surfaces, CaCO₃ and Ca(OH)₂are generated. The residual CaCO₃ and Ca(OH)₂ do not hinderdesulfurization reaction, but an appropriate amount of these componentsare expected to serve to improve this reaction. Further, a metalcomponent of a larger diameter can be removed by magnetic separation orscreening, so that a recovered desulfurizing agent is mainly composed ofa lime component. Furthermore, the grain size of this recovereddesulfurizing agent is restricted by a supply apparatus in adesulfurizing facility where the agent is used. Accordingly, no problemsoccur provided that an appropriate grain size is used. On the otherhand, residual metal of a smaller diameter may remain in the recovereddesulfurizing agent. However, this metal can be reused as an iron sourcefor the subsequent hot metal pretreatment step, thereby significantlycontributing to increasing the yield of iron. Various specific exampleswill be illustrated below.

(i) Crushing Based on Watering Treatment

In this example, desulfurization slag resulting from a desulfurizingstep is simultaneously cooled and crushed using a watering treatment,and is then dried and reused as a desulfurizing agent. Specifically, awatering facility is used to excessively water hot slag resulting fromthe desulfurizing process until the slag is fully impregnated withwater. Subsequently, a drying apparatus is used to completely dry thiswater-containing slag to obtain a desulfurizing agent having a reducedgrain size of 100 mm or smaller. However, a smaller grain size is morepreferable, and the maximum grain size is preferably substantially 30 mmor smaller and more preferably 5 mm or smaller. A mechanical crushingoperation may be performed before or after the watering and dryingsteps. Further, in an actual process, mechanical vibration causes atleast part of desulfurization slag to be crushed during transportation.Specifically, an apparatus used for drying may be a dryer, or a rotarykiln or the like may be used to perform a large-scale drying operation.The size of the apparatus and the like can be set depending on arequired throughput or the like. Any apparatus and method may be usedprovided that water impregnated into the cooled slag can be sufficientlyremoved. The thus recycled desulfurization slag is used as adesulfurizing agent.

(ii) Crushing Based on Watering and Agitating Treatment

In this example, desulfurization slag resulting from the desulfurizingstep is simultaneously cooled and crushed using an appropriate wateringand agitating treatment, and is then reused as a desulfurizing agent.That is, a watering facility is used to uniformly water hot slagresulting from the desulfurizing treatment, while using heavy equipmentsuch as a shovel to agitate the slag. Specifically, a watering operationis performed until the hot slag is cooled down to a temperature of about100° C. The slag is then left cooled to obtain a desulfurizing agent ofa reduced grain size of about 100 mm or smaller. However, a smallergrain size is more preferable, and the maximum grain size is preferablysubstantially 30 mm or smaller and more preferably 5 mm or smaller. Amechanical crushing operation may be performed before or after thewatering step. The cooling target temperature is not limited to aparticular value, but may be set depending on the required throughput orthe like. An appropriate amount of water supplied and agitation reducesthe time required to cool the slag. However, if the watering operationis continued even after the temperature decreases below 100° C., then adrying treatment is required. Thus, the watering operation is desirablystopped before the temperature reaches 100° C. Further, the agitation iscarried out in order to increase cooling speed and make the wateringuniform, so that its frequency may be varied depending on the requiredtreatment time or throughput. The agitation may be omitted.

(iii) Crushing Based on Natural Cooling

In this example, desulfurization slag resulting from the desulfurizingstep is simultaneously cooled and crushed by executing natural cooling.The resultant slag is reused as a desulfurizing agent. That is, hot slagresulting from the desulfurizing step is left as it is so as to maximizethe area of that part of the slag which contacts with air. The slag isthen agitated using heavy equipment such as a shovel. Specifically, arecovered desulfurizing agent of temperature 200° C. or lower and asufficiently reduced diameter can be obtained in three days by spreadingthe hot slag so that its thickness is 0.5 m or smaller and agitating itabout one to three times a day. The thickness of the slag during coolingis not limited to this value, but the target thickness can be setdepending on the required treatment time or throughput, the area of aplace that is available for the recycling treatment, or the like.Further, the agitation is carried out in order to increase the coolingspeed, so that its frequency may also be varied depending on therequired treatment time or throughput. The agitation may be omitted ifmuch treatment time and throughput are available. Furthermore, arecovered desulfurizing agent is similarly obtained if the slag is leftas it is without reducing its thickness.

The cooled and crushed desulfurization slag grains have a maximum grainsize of 100 mm or smaller, preferably 30 mm or smaller, and morepreferably 5 mm. Thus, these grains have a surface area sufficient to beinvolved in a desulfurizing reaction and can be advantageously handledbecause they do not scatter upon addition owing to their larger size. Inaddition, a mechanical crushing operation may also be used.

(iv) Screening Hot Slag

In this example, while hot at 900 to 1,200° C., desulfurization slagresulting from the desulfurizing step is screened using a sieve (30×30mm to 100×100 mm) and is thus separated into metal of a larger diametercontaining a slag component and desulfurization slag of a smallerdiameter. The criterion for the screening is restricted by the supplyapparatus operated when the slag is used as a recovered desulfurizingagent. Typically, the above ranges are appropriate.

After the screening, the desulfurization slag of a reduced diameter isnaturally cooled and reused as it is. Even after the screening, about 20to 30% of Fe component (T.Fe from the slag and metallic Fe) remains.However, this Fe component is introduced into hot metal when it is usedfor the next desulfurizing step, thereby increasing the yield of iron.

When the metal of a larger diameter containing the above slag componentsis cooled, an unreacted lime component in the slag component causes thefollowing reaction:CaO+H₂O=Ca (OH)₂CaO+CO₂=CaCO₃i.e. so-called “powdering” reaction, thereby disintegrating the slagcomponent in the desulfurization slag into a metal component and a slagcomponent. After this step, the above screening operation may beperformed again, so that the desulfurization slag can be efficientlyseparated into large masses of a metal component (containing a smallamount of slag component) and a recovered desulfurizing agent(containing a metal component of a smaller diameter). As a result, about90% of the unreacted lime component in the desulfurization slag isrecovered as a recovered desulfurizing agent.

In this case, any sieve can be used without creating any problemsprovided that it can be operated over a temperature range from 900 to1,200° C.; a sieve made of iron is sufficient. The shape, specification,and the like of this sieve are not limited provided that it can beoperated in a desulfurization slag treatment field. Further, the mesh ofthe sieve is restricted by the supply apparatus in the desulfurizingfacility where the slag is used as a recovered desulfurizing agent. Anappropriate mesh can be used without creating any problems.

As described above, desulfurization slag can be recovered as aninexpensive lime source by exposing an unreacted lime component andseparating large masses of metal from the desulfurization slag withoutmagnetic separation. Furthermore, a recovered desulfurizing agent mainlycomposed of the desulfurization slag recovered after screening still hasa sufficient desulfurizing capability. Therefore, lime can be moreeffectively utilized by reusing the desulfurization slag a number oftimes.

(v) Suppression of Dust

Dried desulfurization slag is very likely to produce dust. However, theinventors note that since desulfurization slag is hot immediately afterbeing produced, most of the slag pieces are massive. That is, a largeamount of dust generated can be prevented by properly handling thedesulfurization slag, e.g. screening it while it is hot after beinggenerated. A higher temperature is more advantageous to suppression ofdust. Slag was actually dropped from a drop height of 3 m with itstemperature varied, and the amount of dust generated was measured. Then,it was found that the amount of dust increases rapidly at a temperatureof 600° C. Further, in connection with an incidental effect of screeningat high temperature, by exposing desulfurization slag to the atmosphereat high temperature, the desulfurization slag can be cooled rapidly andthus powdered. This reduces the subsequent cooling load.

Further, in connection with a facility used for screening, the amount ofdust produced can be reduced using a facility that serves to reduce theheight from which desulfurization slag is dropped vertically. FIG. 12shows the configuration of this facility.

This facility is composed of a diagonally arranged net (12), a diagonalplate on which an undersize slides down, and a sliding way (16). Thisfacility is characterized in that the spacing (13) between the net (12)and the diagonal plate (14) is set at 500 mm or smaller in order toreduce the drop height from which desulfurization slag is dropped.Further, the spacing (15) between the diagonal plate (14) and thesliding way (16) is set at 500 mm or smaller. Furthermore, the facilityis configured so that the drop height from the sliding way (16) to theground (18) is set at 1,500 mm or less. This configuration serves todrastically reduce the amount of dust produced. Thus, the amount of dustproduced during the treatment can also be reduced by using the aboveconfiguration to adjust the grain size (e.g. 70 mm or smaller) of drieddesulfurization slag using the simple facility and low operation costsand further screening the slag while it is at a temperature of 600° C.or higher.

Further, if the desulfurization slag having its grain size adjusted asdescribed above is cooled and then loaded onto a dump truck forrecycling, then naturally, it is powdered faster and is very likely toproduce dust. Thus, desulfurization slag is loaded using a facility suchas the one shown in FIG. 13. A movable basket section (21) is opened inopposite directions and picks up a recovered desulfurizing agent (22).The movable basket section (21) is then closed and carries the picked-uprecovered desulfurizing agent to immediately above a carrier (23) of adump truck, where the movable section (21) is opened. In this case, thedrop height (24) is 1.5 m or less so that the desulfurization slag canbe loaded onto the dump truck while significantly reducing the amount ofdust produced.

(vi) Suppression of Other Dust

If a recovered dry desulfurizing agent is loaded onto a transportvehicle such as a dump truck for recycling as described above, thennaturally, it is powdered faster and is very likely to produce dust.However, the amount of dust produced can be minimized by using a vehiclewith a sucking capability instead of the dump truck to transport therecovered desulfurizing agent. Further, the recovered desulfurizingagent can concurrently be sieved by installing a mesh at a suction portduring suction, thereby enabling efficient handling and transportationwith a reduced amount of dust produced.

Specifically, a suction hose is connected to the vehicle with thesucking capability to suck and load a recovered desulfurizing agentobtained by cooling and crushing desulfurization slag resulting from thehot metal desulfurizing step. At this time, by installing a jig having amesh for screening at the suction port, the desulfurizing agent canconcurrently be sieved so that the required grain size can be obtained.In this case, the jig attached to the suction port during suction may beselected on the basis of the required level of the sieve, i.e. thedesired grain size of the recovered desulfurizing agent.

Furthermore, if no screening operation is required during suction, themesh need not be installed at the suction port, and the desulfurizingagent may be sucked using only the hose.

Moreover, if only large masses of a several tens of cm level must beremoved, a simple jig may be used for suction, jig simply havingpartitions arranged at the tip thereof and formed of metal bars as shownin FIG. 11. An example of such a jig is shown in FIG. 11. This jig issimply composed of a cylinder (1) having the same diameter as that ofthe hose and metal bars (2) attached to the tip of the cylinder so as toform a cone. The size of the mesh of the sieve and the shape of the jigare not limited and are determined depending on the required grain size,pretreatment step, or The like. Furthermore, to facilitate suction, anozzle (3) is attached to the jig to suck air. This sucking jig is usedto suck the recovered desulfurizing agent, thereby simultaneouslyachieving the loading and screening operations.

The cooled and crushed recovered desulfurizing agent grains obtained asdescribed above can solely constitute a desulfurizing agent according tothe present invention or by being combined with another component asrequired. That is, the desulfurizing agent according to the presentinvention may be composed of only the recovered desulfurizing agentgrains obtained through the above described steps or may be formed bycombining these grains with another component such as lime or fluorite,as required. In the latter case, the amount of another component mixedcan be properly determined on the basis of the amount of lime componentcontained in the recovered desulfurizing agent or the requireddesulfurization rate. A preferred determining method will be describedlater.

(Manufacture of Optimum Desulfurizing Agent)

Now, description will be given of a method of manufacturing the optimumdesulfurizing agent using a recovered desulfurizing agent obtained fromdesulfurization slag. Even with the same recovered desulfurizing agent,the desulfurization rate varies significantly depending on the amount ofmaterial added per unit amount of hot metal. However, it has been foundthat the amount of pure CaO component in the recovered desulfurizingagent has a clear correlationship with the amount of materialdesulfurized (the amount of S before treatment−the amount of S aftertreatment).

Thus, even if the components of the recovered desulfurizing agentchange, a stable desulfurization rate is obtained by also introducing apure CaO component into the desulfurizing agent. Then, the ratio of CaOto the recovered desulfurizing agent must be determined. To achievethis, the means described below can be used. Desulfurization slag isroughly divided into a metal component and a slag component. Theinventors note that the ratio of the metal component to the slagcomponent varies significantly, whereas the ratio of CaO to the slagcomponent does not vary significantly. That is, the amount of CaOcontained can be estimated by determining the ratio of the metalcomponent to the slag component in the recovered desulfurizing agent.Furthermore, the metal has a relatively large specific gravity of about7, whereas the slag has a specific gravity of only 2 to 3.

This indicates that bulk density increases in proportion to the amountof metal contained. On the basis of this nature, it is easy to cut out aspecified volume of recovered desulfurizing agent immediately before useand then weigh it. This weight can be easily reflected in the amount ofmaterial introduced. Thus, according to the present invention, theamount of CaO contained in the recovered desulfurizing agent can beestimated promptly and precisely to determine the amount of recovereddesulfurizing agent required for desulfurization.

It has been found that when a desulfurizing agent obtained fromdesulfurization slag is used, even if the components of the recovereddesulfurizing agent change, a stable desulfurization rate is obtained byalso introducing a pure CaO component into the desulfurizing agent.Accordingly, if the recovered desulfurizing agent contains only a smallamount of pure CaO component, the amount of this component added mayincrease. Consequently, the hot metal temperature becomes inappropriate,the amount of slag discharged increases, and other adverse effects areproduced.

Thus, the amount of desulfurizing agent can be reduced by using adesulfurizing agent containing a mixture of a recovered desulfurizingagent with at least one or two of lime (CaO), quick lime (calciumcarbonate CaCO₃), and slaked lime (calcium hydroxide Ca(OH)₂). Ifcalcium carbonate (CaCO₃) and calcium hydroxide (Ca(OH)₂) are added,fine fluxes are generated in molten metal owing to the decompositionreaction shown below. Thus, the reaction surface area increases toimprove the desulfurization reaction rate. On the other hand, thisdecomposition reaction is of an endothermal type and causes oxides to begenerated, which hinder desulfurization reaction, which is of a reducingtype. Accordingly, addition of a large amount of calcium carbonate andcalcium hydroxide may reduce the hot metal temperature or hinder thedesulfurization reaction. Thus, the amount of these components addedmust be 40 wt % or less.

Further, the calcium carbonate component may be added by adjusting, whenlime is calcined, the level of the calcination so as to leave thecalcium carbonate component in the lime. Also in this case, the calciumcarbonate component is mixed with the desulfurizing agent so that thetotal amount of calcium carbonate in the desulfurizing agent is 40 wt %or less. If two or more components are mixed with the desulfurizingagent, the total amount of calcium carbonate component, quick lime, andslaked lime is 40 wt % or less of the total amount of desulfurizingagent. The amount of lime added is not limited and can be freelyadjusted depending on the temperature of the hot metal to be treated.

Furthermore, if these components are added, the same effects areproduced whether predetermined amounts of these components are mixedtogether before addition or are separately added to the hot metal. Inparticular, if the amount of material desulfurized is to be increased ina short time, addition of CaO or the like is more effective if forexample, the temperature of the hot metal to be treated is low.

Further, the desulfurizing capability of the desulfurizing agent canfurther be improved by adding a C source to this recovered desulfurizingagent.

If a C source is added, it acts as a reducing agent to effect thereaction shown below. This facilitates desulfurization to increasedesulfurization reaction efficiency.CaO+S+C→CaS+CO (1)

Further, part of the C source dissolves in the hot metal to increase theamount of heat source that increase the temperature in connection withdecarbonization in a converter. Addition of an excessive amount of Csource may cause an extra amount of C source to remain, the extra amountexceeding the amount of C source that is required for the abovereduction reaction or that needs to dissolve in the hot metal.Consequently, the amount of slag may increase to degrade the quality ofoperations or the environment. Therefore, the amount of C source addedto the desulfurizing agent is desirably 30 wt % or less. Further, the Csource used may contain a sulfur component, so that the concentration ofS in the hot metal may increase depending on the amount of C sourceadded. In particular, the amount of C source dissolving in the hot metalvaries with the temperature of the hot metal. Accordingly, the amount ofC source added can be adjusted on the basis of the type of the C sourceused, the temperature of the hot metal to be treated, the concentrationof S in the hot metal, the required amount of material desulfurized, thetreatment time, or the like. The C source added is not limited, but anyC source may be used such as coal, coke, pitch coke, or plastic.

Furthermore, if these C sources are added, the same effects are producedwhether predetermined amounts of these C sources are mixed togetherbefore addition or are separately added to the hot metal. The C sourcecan be added whether it is massive, granular, or powdery. However, toeasily dissolve in the hot metal, the C source is preferably composed ofpowders of grain size about 1 mm or smaller.

(Method of Desulfurizing Hot Metal)

A desulfurizing agent is applied to a hot metal desulfurizing process,the desulfurizing agent containing the recovered desulfurizing agentobtained as described above and the lime source and/or carbon sourceadded as required. The recovered desulfurizing agent can be appliedirrespective of the configuration of the hot metal desulfurizingfacility or the hot metal desulfurizing method.

The desulfurizing facility using the recovered desulfurizing agent maybe based on mechanical agitation (KR method), the injection method(torpedo), or a converter. The main chemical components of the hot metalare [mass % C]=3.5 to 5.0, [mass % Si]=0 to 0.3, [mass % S]=0.02 to0.05, and [mass % P]=0.1 to 0.15. The temperature of the hot metal isbetween 1,250 and 1,450° C. For a treatment, 5 to 300 tons of hot metalis loaded into a refining container. To mix the desulfurizing agent intothe hot metal, the recovered desulfurizing agent may be mixed with alime component before addition, or the recovered desulfurizing agent andthe lime component may be cut out from separate hoppers before addition.However, to ensure a certain degree of freedom for operations, it iseffective to have a plurality of hoppers. With any refining container,the method used has only to allow the recovered desulfurizing agent andthe lime component to be effectively supplied to a bath surface. Theamounts of recovered desulfurizing agent and lime component introducedare varied depending on the concentrations of Si, S, and P in the hotmetal. The total amount is desirably at most 20 kg/t.

Now, examples of the present invention will be described.

EXAMPLE 1 Example in Which Desulfurization Slag Resulting From a HotMetal Pretreatment Based on the KR Method is Reused as a DesulfurizingAgent for a Hot Metal Desulfurizing Process Based on the KR Method

In this example, desulfurization slag resulting from a desulfurizingstep based on the KR method is positively cooled and crushed by theoptimum treatment method, and is then reused as a desulfurizing agentfor a desulfurizing process based on the KR method. For a recyclingtreatment, a mechanical crushing treatment, a natural cooling treatment,and a watering treatment are properly combined together. Specifically,the recycling treatment is executed by the following method:

Desulfurization slag resulting from a hot metal pretreatment based onthe KR method is first mechanically crushed while hot. Specifically, thedesulfurization slag can be crushed using heavy equipment such as ashovel. Furthermore, the hot slag is watered to facilitate cooling anddisintegration. Specifically, a watering facility is used for cooling.

Alternatively, the desulfurization slag may be cooled by leaving it asit is without watering. In this case, to facilitate cooling, thedesulfurization slag may be spread as thin as possible to increase thecontact area thereof which contacts with the atmosphere. Furthermore,reaction of the desulfurization slag with steam or carbon dioxide in theatmosphere may be promoted to facilitate disintegration of a limecomponent and generation of a compound such as calcium carbonate orcalcium hydroxide. Moreover, it is possible to crush the hot slag andthen pass it through a sieve to separate large masses such as metaltherefrom.

The desulfurization slag resulting from this method was positivelytreated to facilitate cooling and crushing of the desulfurization slag,thereby generating recovered desulfurizing agent grains of maximum grainsize 30 mm or smaller. The grains may further be mechanically crushed.

Since the recovered desulfurizing agent obtained has a maximum grainsize of 30 mm or smaller, a surface area is obtained which is enough tobe involved in the desulfurization reaction.

Since the grains are appropriately fine, production of dust can beprevented. Further, during reuse with the KR method, a decrease in yieldcaused by scattering can be prevented. Furthermore, the grains can beproperly caught in waves in a bath.

Moreover, generation of a compound such as calcium carbonate or calciumhydroxide can be facilitated by positively promoting cooling anddisintegration as described above. When added to the hot metal, thesecompounds are decomposed while undergoing dehydrating and degassingreaction, to facilitate agitation of the hot metal. Further, thedecomposition increases the reaction surface area, thereby improving thedesulfurization efficiency.

The above recycling method efficiently provides a desulfurizing agent ofgrain size 30 mm or smaller having a new surface with a desulfurizingcapability. This desulfurizing agent was used in the example.

For comparison, a conventional desulfurizing agent was used whichcontained 90% of lime and about 5% of fluorite. Table 1 shows theaverage compositions of the conventional desulfurizing agent as acomparative example and the recovered desulfurizing agent as thisexample.

These desulfurizing agents were applied to a mechanical agitation typedesulfurizing apparatus under the conditions shown in Table 2 to carryout a hot metal desulfurization pretreatment.

After the hot metal desulfurization, the desulfurization rate achievedby the mechanical agitation type desulfurizing apparatus was checked foreach of the desulfurizing agents. FIG. 3 shows the relationship betweenthe amount of additives (fluxes) per unit amount of hot metal and thedesulfurization slag rate. In the graph of FIG. 3, a curve a indicatesresults for the desulfurization slag of the present invention, while acurve b indicates results for the conventional desulfurizing agent as acomparative example.

The graph of FIG. 3 indicates that with an equal amount of desulfurizingagent added per unit amount of hot metal, the desulfurization rate ofthe recovered desulfurizing agent is about 50 to 90% of that of thecomparative desulfurizing agent. Furthermore, FIG. 4 shows comparisonbased on the amount of lime component added per unit amount of hotmetal. This graph indicates that in terms of a contained lime component,the recovered desulfurizing agent has a desulfurizing capabilitysubstantially equivalent to that of the comparative desulfurizing agent.Thus, a treatment with the recovered desulfurizing agent is expected toproduce a desulfurizing effect equivalent to that of a desulfurizingtreatment with lime provided that the amount of lime added per unitamount of hot metal, in the recovered desulfurizing agent added, isestimated to be equivalent to the amount of lime required for thedesulfurization reaction.

Table 3 shows changes in the amount of lime used before and afterintroduction of the present process. The table indicates that the reuseof desulfurization slag reliably reduced the amount of lime used,thereby reducing treatment costs by about 40% compared to the costsrequired before introduction.

Table 4 shows changes in the amount slag generated before and afterintroduction of the present process. The amount of slag generateddecreased by 3,000 t/month, thereby demonstrating that the presentprocess reduces not only desulfurization costs but also the amount ofslag to solve environmental problems.

This example produces significant effects. For example, it reduces thedesulfurization costs, allows desulfurization slag to be reused, andreduces the amount of slag generated to solve environmental problems.Therefore, this example has a high industrial value.

EXAMPLE 2 Example Relating to the Desulfurization Rate of RecycledDesulfurization Slag and the Number of Times That the DesulfurizationSlag is Recycled

In this example, desulfurization slag resulting from a desulfurizingstep was cooled and crushed by a natural cooling or watering treatmentand then reused as a desulfurizing agent during the same process. Arecovered desulfurizing agent of grain size 100 mm or smaller andtemperature 200° C. or lower was obtained without mechanical crushing,and was used as a desulfurizing agent according to this example.Furthermore, the recovered desulfurizing agent was used to carry outdesulfurization. The resultant slag was recovered again, and the abovemethod was used to set its size and temperature at 100 mm or smaller and200° C. or lower, respectively. Then, the slag was reused as adesulfurizing agent according to this example.

For comparison, the conventional desulfurizing agent was used whichcontained 90% of lime and about 5% of fluorite. Table 1 shows theaverage compositions of the recovered desulfurizing agent andconventional desulfurizing agent used (all tables are shown at the endof the specification).

These desulfurizing agents were applied to the mechanical agitation typedesulfurizing apparatus under the conditions shown in Table 2 to carryout a hot metal desulfurization pretreatment.

FIG. 1 shows the relationship between the amount of lime added per unitamount of hot metal and the desulfurization rate at each desulfurizationlevel. This figure indicates that the recovered desulfurizing agentsresulting from the first and second recycling operations had adesulfurizing capability equal to 80% or more of that of the comparativedesulfurizing agent. Thus, even with a treatment using the recovereddesulfurizing agent resulting from the second recycling operation,properly increasing the amount of lime added is expected to serve toproduce a desulfurizing effect equivalent to that of a desulfurizingtreatment with lime.

FIG. 2 shows changes in the amount of desulfurizing agent used beforeand after introduction of the present process, in connection with thenumber of desulfurizing agent recycling operations and changes in theamount of recovered desulfurizing agent used. The figure indicates thatrecycling the desulfurizing agent a number of times served todrastically reduce the amount of recovered desulfurizing agent used.When a recycling operation was preformed about three times, the amountof desulfurizing agent used decreased by about 75% compared to theamount measured before introduction. At the same time, the amount ofslag generated decreased drastically, thereby demonstrating that thepresent process is also effective on environmental problems.

Thus, according to this example, the use of the desulfurizing agent ofthe present invention reduces desulfurization costs, allowsdesulfurization slag to be recycled a number of times, and reduces theamount of waste to solve environmental problems.

EXAMPLE 3 Cooling and Crushing Based on a Watering Treatment

In this example, desulfurization slag resulting from a desulfurizingstep is simultaneously cooled and crushed by a watering treatment, andis then dried and reused as a desulfurizing agent. Specifically, awatering facility is used to excessively water the hot slag subjected tothe desulfurizing treatment until the slag is completely impregnatedwith water. Subsequently, a drying apparatus is used to completely drythe water-containing slag to obtain a desulfurizing agent of asufficiently reduced grain size of about 5 mm or smaller. Specifically,the apparatus used for drying may be a dryer, or a rotary kiln or thelike may be used to perform a large-scale drying operation. The size ofthe apparatus and the like can be set depending on the requiredthroughput or the like. Any apparatus and method may be used providedthat water impregnated into the cooled slag can be sufficiently removed.

The thus recycled desulfurization slag was used as a desulfurizing agentaccording to this example.

For comparison, the conventional desulfurizing agent was used whichcontained 90% of lime and about 5% of fluorite. Table 5 shows theaverage compositions of the recovered desulfurizing agent andconventional desulfurizing agent used.

These desulfurizing agents were applied to the mechanical agitation typedesulfurizing apparatus under the conditions shown in Table 2 to carryout a hot metal desulfurization pretreatment.

FIG. 5 shows the relationship between the amount of lime added per unitamount of hot metal and the desulfurization rate at each desulfurizationlevel. This figure indicates that with an equal amount of limeintroduced per unit amount of desulfurization slag, the recovereddesulfurizing agent (indicated by a solid line) has a desulfurizingcapability equal to about 70% of that of the comparative desulfurizingagent (indicated by a broken line).

EXAMPLE 4 Cooling and Crushing Based on a Watering Treatment

In this example, desulfurization slag resulting from a desulfurizingstep is simultaneously cooled and crushed by a watering treatment, andis then reused as a desulfurizing agent. That is, the watering facilityis used to uniformly water the hot slag resulting from the desulfurizingtreatment, while using heavy equipment such as a shovel to agitate theslag. Specifically, the watering treatment is continued until the hotslag is cooled down to a temperature of 80 to 150° C. Subsequently, theslag is left and cooled until its temperature reaches the roomtemperature. Thus, a desulfurizing agent is obtained which has asufficiently reduced grain size of 5 mm or smaller. This target coolingtemperature is not limited to a certain value, but can be set dependingon the required throughput or the like.

For comparison, the conventional desulfurizing agent was used whichcontained 90% of lime and about 5% of fluorite. Table 7 shows theaverage compositions of the recovered desulfurizing agent andconventional desulfurizing agent used.

These desulfurizing agents were applied to the mechanical agitation typedesulfurizing apparatus under the conditions shown in Table 2 to carryout a hot metal desulfurization pretreatment.

FIG. 9 shows the relationship between the amount of lime added per unitamount of hot metal and the desulfurization rate at each desulfurizationlevel. This figure indicates that with an equal amount of lime added perunit amount of hot metal, the desulfurizing agent recovered by wateringthe slag until its temperature reaches 150° C. has a desulfurizingcapability substantially equivalent to that of the comparativedesulfurizing agent.

However, even with the same recovered desulfurizing agent subjected tothe watering treatment, the desulfurizing capability varies depending onwhether the watering treatment is stopped or continued after thetemperature reaches 100° C. This is assumed to be because the amount ofcalcium hydroxide generated in the recovered desulfurizing agentincreases significantly if the slag is cooled even after its temperaturereaches 100° C. and because such an excessive increase in the amount ofcalcium hydroxide affects desulfurization, as shown in FIG. 7.

In this regard, FIG. 6 shows the relationship between the temperature ofthe slag measured at the end of watering and the time required to cool40 T of desulfurization slag down to the room temperature. This figureindicates that more time is required to cool the slag down to the roomtemperature as the temperature of the slag is higher at the end ofwatering.

The desulfurization treatment method of this example enablesdesulfurization slag to be efficiently treated, reduces the time andcost required to recover a desulfurizing agent, and allows a largeamount of desulfurization slag to be recycled to reduce the amount ofslag, thereby helping solve environmental problems.

EXAMPLE 5 Cooling and Crushing Based on a Natural Cooling Treatment

In this example, desulfurization slag resulting from a desulfurizingstep is simultaneously cooled and crushed by a watering treatment, andis then reused as a desulfurizing agent. Specifically, the hot slagsubjected to the desulfurization treatment is left as it is so as tomaximize the contact area thereof which contacts with the atmosphere,and is then agitated using heavy equipment such as a shovel.Specifically, a recovered desulfurizing agent of temperature 200° C. orlower and a sufficiently reduced diameter can be obtained in three daysby spreading the hot slag so that its thickness is 0.5 m or smaller andagitating it about one to three times a day. The thickness duringcooling is not limited to this value, but the target thickness can beset depending on the required treatment time or throughput, the area ofa place that is available for the recycling treatment, or the like.Further, the agitation is carried out to increase the cooling speed, sothat its frequency may also be varied depending on the requiredtreatment time or throughput. The agitation may be omitted if muchtreatment time and throughput are available. This desulfurizing agentwas used as one according this example.

Furthermore, a recovered desulfurizing agent is similarly obtained ifthe slag is left as it is without reducing its thickness. This is alsoshown as a desulfurizing agent according to this example.

Further, for comparison in terms of the cooling method, mechanicallycrushed desulfurization slag (a mechanically crushed agent) was used.

Furthermore, for comparison in terms of desulfurizing behavior, theconventional desulfurizing agent was used which contained 90% of limeand about 5% of fluorite. Table 8 shows the average compositions of therecovered desulfurizing agent and conventional desulfurizing agent used.

These desulfurizing agents were applied to the mechanical agitation typedesulfurizing apparatus under the conditions shown in Table 2 to carryout a hot metal desulfurization pretreatment.

FIG. 10 shows the relationship between the amount of lime added per unitamount of hot metal and the desulfurization rate at each desulfurizationlevel. This figure indicates that with an equal amount of lime added perunit amount of hot metal, the desulfurizing agent recovered by executingnatural cooling has a desulfurizing capability substantially equivalentto that of the comparative desulfurizing agent.

Further, even with the same non-watering treatment, the desulfurizationcapability of the mechanically crushed recovered desulfurizing agent isinferior to that of the recovered desulfurizing agent which has beennaturally cooled. This is assumed to be because the recovereddesulfurizing agent which has been naturally cooled contains severalpercent of calcium carbonate generated therein as shown in Table 8 andbecause during the desulfurizing treatment, decomposition of calciumcarbonate facilitates agitation of the hot metal and increases thereaction surface area to improve the desulfurization efficiency.

Comparison of the desulfurizing capability of the recovereddesulfurizing agent and the time required for the recycling treatmentwill be shown below in connection with the different methods ofrecovering desulfurization slag shown in Examples 3 to 5.

Even with the same recovered desulfurizing agent, a higherdesulfurization capability is obtained by crushing slag by watering (at150° C.) than by mechanically crushing it. This is assumed to be becausethe recovered desulfurizing agent crushed by watering contains severalpercent of calcium carbonate generated therein and because during thedesulfurizing treatment, decomposition of calcium carbonate facilitatesagitation of the hot metal and increases the reaction surface area toimprove the desulfurization efficiency.

However, with the same recovered desulfurizing agent subjected to thewatering treatment, the desulfurization capability varies depending onwhether the watering operation is controlled (150° C.) or is continueduntil the desulfurizing agent is completely impregnated with water. Thisis assumed to be because cooling the desulfurization slag down to atemperature of 100° C. or lower significantly increases the amount ofcalcium hydroxide (Ca(OH)₂) generated in the recovered desulfurizationslag and because such an excessive increase in the amount of calciumhydroxide affects desulfurization.

Further, Table 6 shows a comparison of the treatment conditions for thedesulfurization slag and the time required to treat about 40 T ofdesulfurization slag. This table indicates that the watereddesulfurization slag is cooled much faster than that left as it iswithout undergoing watering. The table also indicates that thewatering-based treatment allowed the slag to be perfectly crushed withinthe time required for cooling. Furthermore, controlling the amount ofwater supplied eliminates the need for a drying treatment after thewatering treatment, thereby saving the facilities, cost, and timerequired for crushing and drying. Further, watering during the treatmentprevents production of dust from the desulfurization slag being treated.

On the other hand, with the natural cooling treatment for the slagwithout watering, the required treatment Lime is 170 hours if thethickness of the desulfurization slag is 1.5 m but decreasesdramatically down to 70 hours if the thickness is 0.4 m. In this regard,FIG. 8 shows the relationship between the treatment time and thetemperature for the different thicknesses for the respective treatments(the solid line indicates a thickness of 0.4 m, while the broken lineindicates a thickness of 1.5 m). The treatment speed increases withdecreasing thickness. However, the thickness is determined on the basisof the required cooling speed, the area of a place available forcooling, or the like. The non-watering treatment method according to thepresent invention requires slightly more time than the wateringtreatment, but does not require any watering facilities. Further, withthe watering treatment, it is difficult to uniformly water the slag inthe case of bulk treatment, possibly resulting in water-containing slag.It is thus difficult to provide such control that such slag is notgenerated. Problems with the water-containing slag are that it isdifficult to handle after the recovery treatment, that flame isgenerated when it is introduced, that it has an inadequate desulfurizingcapability, and the like. On the other hand, the non-watering naturalcooling treatment method allows the slag to be easily uniformly treatedeven in the case of bulk treatment. This method also causes the limecomponent in the slag to be “powdered” (part of the lime component inthe slag reacts with moisture or carbon dioxide in the atmosphere duringthe natural cooling operation to change the volume of the slag, whichthus becomes powdery), so that the grain size of the slag decreases asthe time elapses. This eliminates the need for mechanical crushing afterthe cooling treatment. Furthermore, part of the lime component in theslag reacts with carbon dioxide in the atmosphere to generate calciumcarbonate, which is effective in improving the desulfurization reactionefficiency.

As described above, by effectively using the desulfurization slagtreatment method of this example according to the conditions such as theamount of desulfurization slag to be recovered and the availablefacilities, the desulfurization slag can be inexpensively andefficiently treated to reduce the time and cost required to recover thedesulfurization slag. This in turn allows a large amount ofdesulfurization slag to be recycled and drastically reduces the amountof slag to solve environmental problems.

EXAMPLE 6 Screening Desulfurization Slag After Separation of Metal

In this example, while hot at 900 to 1,200° C., desulfurization slagresulting from a desulfurizing step is screened using a screeningapparatus 12 with a □70-mm mesh, and is thus separated intodesulfurization slag of a smaller diameter containing a large amount ofunreacted lime and metal of a larger diameter. In this case, any sievecan be used without creating any problems provided that it can beoperated over a temperature range from 900 to 1,200° C.; a sieve made ofiron is sufficient. The shape, specification, and the like of this sieveare not limited provided that it can be operated in a desulfurizationslag treatment field. Further, the mesh of the sieve is restricted bythe supply apparatus in the desulfurizing facility where the slag isused as a desulfurizing agent. An appropriate mesh can be used withoutcreating any problems.

The sieved desulfurization slag of a small diameter is cooled and thentransported, as a recovered desulfurizing agent, to the supply apparatuson the desulfurizing facility, where it is used.

On the other hand, the residual metal can be easily recovered using theabove described method. Further, this metal can then be reused as aniron source for the hot metal pretreatment step, thereby significantlycontributing to increasing the yield of iron.

In this example, desulfurization slag was recovered under conditionssuch as those shown in Table 9. The sieve was angularly installedrelative to a horizontal surface. Hot slag was screened by dropping itfrom the top of the sieve. Table 10 shows the mass balance of adesulfurizing agent during its recovery process. It has been confirmedthat even if the desulfurization slag resulting from the desulfurizingstep is sieved, about 90% of CaO in the desulfurization slag isrecovered, with the metal component effectively removed. That is, themethod of the present invention has proved to enable the slag componentin the desulfurization slag to be effectively recovered without magneticseparation.

The recovered desulfurizing agent and the lime were applied to thedesulfurizing treatment, and desulfurizing behavior was checked. Forsome levels, the mixture of the recovered desulfurizing agent with thelime was carried out in two manners. That is, these components weremixed together and then loaded into a hopper and then a certain part ofthe mixture was cut out, or a recovered desulfurizing agent and a limecomponent were cut out from the respective hoppers. Table 17 also showsthe average composition of a lime-based desulfurizing agent as aconventional example. These desulfurizing agents were applied to themechanical agitation type desulfurizing apparatus under the testconditions shown in Table 11 to carry out hot metal desulfurization.

FIG. 16 shows the relationship between the amount of lime added per unitamount of hot metal and the desulfurization rate. The concentration ofthe lime component in the recovered desulfurization slag is lower thanthat of the comparative example, so that the amount of desulfurizingagent introduced is larger in this example than in the comparativeexample. However, the desulfurizing capability of this example isequivalent to that of the comparative example, thereby demonstrating theeffectiveness of the present invention.

Then, to examine the effects of the temperature of the desulfurizationslag on dust, the hot slag was left as it was for 30 minutes to 4 hoursand was then passed through the screening apparatus 12, previouslydescribed. Then, it was checked how dust was produced and how thetemperature changed. Table 14 shows how dust was produced when the slagwas screened with its temperature changed. The table indicates thatproduction of dust changed significantly after pre-screening temperaturereached 600° C. This nature can be utilized to perform a screeningoperation or the like without any dust collectors. Further, Table 15shows the result of measurement of a decrease in temperature during thescreening operation. During screening, the temperature decreases byabout 100° C., thus reducing the time required for cooling. Further,Table 16 shows how dust was produced in the screening facility accordingto the present invention and in a screening facility that does not havethe diagonal plate (14) or sliding way (16), shown in FIG. 12.

With the screening operation according to the present invention, evendesulfurization slag that is likely to produce dust at a temperature of600° C. or lower can be screened without any dust collectors. The methodof this example significantly simplify a facility which is used torecycle desulfurization slag and which adjusts the grain size of drydesulfurization slag. Further, production of dust can be preventedduring screening using a simple facility. Furthermore, during screening,the slag efficiently contacts with the atmosphere and is thusefficiently cooled. Moreover, by loading the desulfurization slag onto adump truck using the loading method described herein, the loadingoperation can be achieved while preventing production of dust withoutany dust collectors.

The treatment of this example enables the desulfurizing agent to beinexpensively and efficiently treated and reduces the amount ofdesulfurization slag to be treated compared to the prior art, therebydrastically reducing costs. Other significant effects of this exampleare the possibility of recovering and recycling a large amount ofdesulfurization slag and a decrease in the amount of slag, which leadsto solution of environmental problems.

EXAMPLE 7 Handling of a Recovered Desulfurizing Agent

If a recovered dry desulfurizing agent is loaded onto a dump truck forrecycling, then naturally, it is powdered faster and is very likely toproduce dust. However, the amount of dust produced can be minimized byusing a vehicle with a sucking capability instead of the dump truck totransport the recovered desulfurizing agent. Further, the recovereddesulfurizing agent can concurrently be screened by installing a mesh ata suction port during suction, thereby enabling efficient handling andtransportation with a reduced amount of dust produced.

Specifically, a suction hose is connected to the vehicle with thesucking capability to suck and load a recovered desulfurizing agentobtained by cooling and crushing desulfurization slag resulting from thehot metal desulfurizing step. At this time, by installing a jig having amesh for screening at the suction port, the desulfurizing agent canconcurrently be screened so that the required grain size can beobtained. In this case, the jig attached to the suction port duringsuction may be selected on the basis of the required level of the sieve,i.e. the desired grain size of the recovered desulfurizing agent.

Furthermore, if no screening operation is required during suction, themesh need not be installed at the suction port, and the desulfurizingagent may be sucked using only the hose.

Moreover, if only large masses of a several tens of cm level must beremoved, the recovered desulfurization slag was simultaneously loadedand sieved on the basis of the sucking operation using a simple jig onlyhaving partitions arranged at the tip thereof and formed of metal barsas shown in FIG. 11. In this example, desulfurization slag resultingfrom the mechanical agitation type desulfurizing step was used. Thedesulfurization slag was left to be naturally cooled in a building forabout 6 days. During this period, to facilitate cooling, thedesulfurization slag was agitated about three times a day.

The hose used for suction had a diameter of 15 cm, and the jig 11attached to the suction port during suction included two nets with a 30-and 5-mm meshes and a conical jig. These jigs all had an air suctionport. Furthermore, as a comparative example, the desulfurization slagwas sucked using only the hose and without any jigs. Further, Table 12shows the specification of the suction vehicle used.

Table 13 shows throughput and suction performance as results for suctionof a recovered desulfurizing agent using the various types of suctionports previously described. These results indicate that the recovereddesulfurizing agent can be efficiently loaded at a rate of about 1t/min. while being simultaneously sieved, whichever suction port isused. Further, it has been confirmed that during suction, thetemperature of the recovered desulfurizing agent decreases by about 30°C. after the treatment as a result of the positive contact of thedesulfurizing agent with the atmosphere.

Furthermore, most of the sieved large masses and the slag on the sieveare metal and can thus be used for another process.

It has been ascertained that with the handling method of the presentinvention, even a recovered desulfurizing agent which is composed offine powders and which is likely to produce dust can be simultaneouslyand efficiently sieved and loaded onto a transport vehicle without usingany dust collectors.

In this example, with the desulfurizing agent treating method used torecycle desulfurization slag, a transport vehicle with a suckingcapability can be used to load a recovered desulfurizing agent whileminimizing the amount of dust produced. Furthermore, the recovereddesulfurizing agent can be efficiently treated by simultaneouslyperforming a sucking and screening operations. Moreover, the recovereddesulfurizing agent efficiently contacts with the atmosphere or suctionair and is thus efficiently cooled.

EXAMPLE 8 Measurement of the Content of CaO in Desulfurization Slag

In this example, desulfurization slag resulting from a mechanicalagitation type desulfurizing step was used. The desulfurization slag waspassed through the screening apparatus composed of a net with a □70-mmmesh.

The following are the results of the reuse, in a mechanical agitationtype desulfurizing facility, of the recovered desulfurizing agent havingits grain size adjusted. FIG. 14 shows the relationship between theamount of pure CaO component and the amount of material desulfurized ΔS(the amount of sulfur before treatment (S)−the amount of sulfur aftertreatment (S)). It has been found that the amount of pure CaO componenthas a clear correlationship with the amount of material desulfurized.This nature can be utilized to easily determine the amount ofdesulfurizing agent introduced depending on the amount of sulfur beforetreatment (S), thereby achieving a stable desulfurization rate.

Further, FIG. 15 shows the relationship between the bulk density and theCaO mass %. This figure indicates that the bulk density decreases withincreasing amount of CaO. This tendency can be utilized to estimate theCaO mass % of the recovered desulfurizing agent by measuring the bulkdensity thereof.

According to this example, when desulfurization slag was reused in themechanical agitation type desulfurizing facility, the required amount ofdesulfurizing agent could be determined promptly, while achieving astable desulfurization rate.

EXAMPLE 9 Example Relating to Improvement of Desulfurization Efficiencyby Addition of a Lime Source (CaO, CaCO₃, Ca(OH)₂)

Desulfurization slag resulting from a desulfurizing step was cooled andcrushed by a natural cooling or watering treatment to obtain adesulfurizing agent (hereinafter referred to as a “recovereddesulfurizing agent”). One or more of the above-described desulfurizingagents were added to the recovered desulfurizing agent, which was thenused as a desulfurizing agent according to this example. In acomparative example, only the recovered desulfurizing agent was used.Table 18 shows the amount of recovered desulfurizing agent added, thetypes of desulfurizing agents, and the amounts of desulfurizing agentsadded.

These desulfurizing agents were applied to the mechanical agitation typedesulfurizing apparatus under the test conditions shown in Table 19 tocarry out hot metal desulfurization.

FIG. 19 shows the relationship between the amount of lime componentadded per unit amount of hot metal and the desulfurization rate at eachdesulfurization level. This figure indicates that with an equal amountof lime added per unit amount of hot metal, the use of the (d) recovereddesulfurizing agent+lime slightly increases the desulfurization ratecompared to the use of (a) only the recovered desulfurizing agent. TheFIG. also indicates that the use of the (b) recovered desulfurizingagent+CaCO₃, the (c) recovered desulfurizing agent+CaCO₃+Ca(OH)₂, or the(e) recovered desulfurizing agent+lime+CaCO₃+Ca(OH)₂ increases both theamount of CaCO₃ and Ca(OH)₂ added and the desulfurization rate comparedto the use of (a) only the recovered desulfurizing agent. This isassumed to be because during the desulfurizing treatment, decompositionof CaCO₃ or Ca(OH)₂ increases the reaction surface area or facilitatesagitation of hot metal to improve the desulfurization efficiency, aspreviously described.

Table 20 shows the ratio of CaO to the recovered desulfurizing agent,the amount of recovered desulfurizing agent added, the types ofdesulfurizing agents, the amounts of desulfurizing agents added, and thetotal amount of desulfurizing agents which are measured if 8 kg-CaO/T oflime component is contained in the desulfurizing agents of thecompositions used in this example. If each desulfurizing agent requiresthe same amount of lime component, when only the recovered desulfurizingagent is used, a large amount of desulfurizing agent is required if therecovered desulfurizing agent contains only a small amount of pure limecomponent. However, addition of lime and calcium carbonate or calciumhydroxide improves the desulfurization slag rate and reduces the amountof desulfurizing agent used. This reduces the amount of slag dischargedafter the treatment. Furthermore, the compositions of the desulfurizingagents can be freely adjusted according to the state of the hot metal tobe treated or the required treatment conditions. Moreover, similareffects are produced even if these desulfurizing agents are separatelyadded to the hot metal instead of being mixed together.

EXAMPLE 10 Example Relating to Improvement of Desulfurization Efficiencyby Addition of Various Carbon Sources

Desulfurization slag resulting from a desulfurizing step was cooled andcrushed by a natural cooling or watering treatment to obtain adesulfurizing agent (hereinafter referred to as a “recovereddesulfurizing agent”). Various C sources were added to the recovereddesulfurizing agent, which was then used as a desulfurizing agentaccording to this example. Table 21 shows the amount of recovereddesulfurizing agent added, the types of desulfurizing agents, and theamounts of desulfurizing agents added. The C sources were composed ofpowders of grain size 1 mm or smaller.

These desulfurizing agents were applied to the mechanical agitation typedesulfurizing apparatus under the test conditions shown in Table 22 tocarry out hot metal desulfurization.

FIG. 20 shows the relationship between the amount of lime componentadded per unit amount of hot metal and the desulfurization rate at eachdesulfurization level. This figure indicates that with an equal amountof lime added per unit amount of hot metal, the use of the (d) to (d)recovered desulfurizing agent+C source increases the desulfurizationrate linearly with the amount of C source added, compared to the use of(a) only the recovered desulfurizing agent. It has also been confirmedthat equivalent effects are produced regardless of the type of the Csource or the method of adding the C source. This is assumed to bebecause the C source acts as a reducing agent during the desulfurizingstep to improve the desulfurization rate, as previously described.Further, part of the C source dissolved in the hot metal to increase theconcentration of C in the hot metal by 0.1 to 0.5% in each case underthe conditions of this example.

By thus adding a C source such as anthracite or coke, thedesulfurization rate can be improved while reducing the amount of slagdischarged after the treatment. Any C source may be used, such as coal,coke, pitch coke, or plastic. Furthermore, the compositions of thedesulfurizing agents can be freely adjusted according to the state ofthe hot metal to be treated or the required treatment conditions.Moreover, similar effects are produced even if these desulfurizingagents are separately added to the hot metal instead of being mixedtogether.

As described above, the present invention provides a method ofdesulfurizing hot metal, which method effectively reuses desulfurizationslag resulting from a hot metal desulfurizing treatment in order toreduce the costs of hot metal desulfurization and the amount of slaggenerated. Further, the present invention provides a desulfurizing agentthat allows a hot metal desulfurizing treatment with a reduced amount ofslag generated to be inexpensively carried out. The present inventionproduces significant effects. For example, it reduces desulfurizationcosts, allows desulfurization slag to be recycled, and reduces theamount of slag to solve environmental problems. Therefore, the presentinvention has a high industrial value. TABLE 1 Main components ofaverage conventional desulfurizing agent and recovered desulfurizingagent Main component T-Fe CaO SiO₂ Al₂O₃ T-S CaF₂ (1) Conventional —90.0 — — — 5.0    desulfurizing agent (2) Recovered desulfurizing 19.253.6 9.3 4.1 1.7 2.1    agent (3) Desulfurization slag 19.7 45.0 12.011.0 3.2 2.2    resulting from    recovered desulfurizing    agent-used   defulfurizing treatment (unit: mass %)

TABLE 2 Test conditions Item Contents Desulfurizing method Mechanicallyagitating type desulfurizing apparatus Treatment Container Hot metalladle Throughput   140-160 t Hot metal temperature  1300-1450° C. Hotmetal [S]  0.05-0.02% Amount of 0 to 30 kg - CaO/t desulfurizing agent

TABLE 3 KR desulfurization treatment conditions After Beforeintroduction introduction of recycling of recycling Item process processHot metal [S] 0.05-0.02% 0.05-0.02% Amount of 8 kg/t   3-5 kg/t limeused Amount of — 8 kg - CaO/t desulfurizing agent used

TABLE 4 Changes in amount of slag generated associated with introductionof desulfurization recycling process Amount of slag generated (t/month)Before recycling 8000 After recycling 4000 Amount of decrease in 4000amount of slag generated

TABLE 5 Main components of average conventional desulfurizing agent andrecovered desulfurizing agent Main Conventional Drying after componentdesulfurizing agent watering CaO 90.0 53.6 T-Fe — 19.2 SiO₂ — 9.3 Al₂O₃— 4.1 T-S — 1.7 CaF₂ 5.0 2.1 CaCO₃ — 2.3 Ca(OH)₂ — 18.3 (Unit: mass %)

TABLE 6 Time required for desulfurizing treatment method and 40Ttreatment Non-watering Watering Natural cooling Drying DesulfurizationDesulfurization Treatment Watering after slag thickness slag thicknessMechanical method 150° C. 80° C. watering 1.5 m 0.4 m crushing Amount ofwater 8 15 30 — — supplied (t) Mechanical Not required Not Not requiredRequired crushing required Drying Not required Required Not required Notrequired Time required 30 24 10 170 70 50 to cool slag down to 100° C.

TABLE 7 Main components of average conventional desulfurizing agent andrecovered desulfurizing agent Conventional watered watered Maindesulfurizing agent agent component agent (150° C.) (80° C.) CaO 90.053.6 53.6 T-Fe — 19.2 19.2 SiO₂ — 9.3 9.3 Al₂O₃ — 4.1 4.1 T-S — 1.7 1.7CaF₂ 5.0 2.1 2.1 CaCO₃ — 5.2 1.1 Ca(OH)₂ — 3.3 24.1 (Unit: mass %)

TABLE 8 Main components of average conventional desulfurizing agent andrecovered desulfurizing agent Non- watered and Non-watered Conventionalnaturally mechanically Main desulfurizing cooled crushed component agentagent agent CaO 90.0 53.6 53.6 T-Fe — 19.2 19.2 SiO₂ — 9.3 9.3 Al₂O₃ —4.1 4.1 T-S — 1.7 1.7 CaF₂ 5.0 2.1 2.1 CaCO₃ — 6.5 0.4 Ca(OH)₂ — 2.5 0.5(Unit: mass %)

TABLE 9 Recovery treatment conditions for desulfurization slagDesulfurization 600-1200° C. slag temperature Sieve Made of iron, mesh70 mm × 70 mm Sieve angle  8-15° Throughput 2 t/times

TABLE 10 Changes in composition during recovery process Conventional Hotslag Recovered desulfurizing (before desulfurizing agent screening)agent Weight Metal 0 20 7.2 ratio component % Slag 100 80 92.8 component% Slag CaO 90 54 53.8 composition T-Fe — 18.5 18.2 SiO₂ — 9.0 8.8 Al₂O₃— 4.1 4.1 T-S — 1.7 1.8 CaF₂ 5 2.1 1.8 CaCO₃ — 0.4 1.3 Ca(OH)₂ — 0.5 1.0(unit: mass %)

TABLE 11 Test conditions Item Contents Desulfurizing Mechanicallyagitating method type desulfurizing apparatus Refining Hot metal ladlecontainer Throughput   130-165 t Hot metal  1300-1450° C. temperatureInitial [S] in  0.02-0.05% hot metal Agitation time 5 to 7 minutesAmount of    6-30 kg/t desulfurizing agent

TABLE 12 Specification of suction vehicle Suction air quantity 80 m³Vacuum pressure −650 mmHg Suction tank capacity 5.5 m³ Connection hosediameter 15 cm

TABLE 13 Sucking method and Recovered desulfurizing agent loadingcapability Amount of Sucking method recovered Suction Sucking Diameterof Suction desulfurizing agent time capability hose used port typesucked (t) (min.) (t/min.) 15 cm 5 mm mesh 14.2 14.7 1.00 15 cm 30 mmmesh 22.4 20.13 1.11 15 cm Cone-shaped 6.00 5.10 1.18 15 cm Cone-shaped16.2 16.2 1.00 15 cm Cone-shaped 11.1 12.5 0.89 15 cm Cone-shaped 29.733.2 0.89 15 cm Cone-shaped 15.0 15.9 0.94 15 cm Only hose 13.8 9.8 1.40

TABLE 14 Screening temperature and dust generation Temperature beforescreening Dust generation 420° C. Large 610° C. Small 940° C. Verylittle

TABLE 15 Decrease in temperature during screening Before After Decreasein screening screening temperature 1 930° C. 790° C. Δ 140° C. 2 940° C.880° C.  Δ 60° C. 3 940° C. 860° C.  Δ 80° C.

TABLE 16 Dust generation observed when present facility is used SlagDust temperature generation Present screening 450° C. Little facilityScreening facility 480° C. Very large without diagonal plate or slidingway

TABLE 17 Average desulfurizing agent composition used when bothrecovered desulfuring agent and lime are used Recovered desulfurizingagent + 50% lime Conventional Recovered Recovered (introduced Recovereddesulfurizing Recovered desulfurizing desulfurizing throughdesulfurizing agent desulfurizing agent + 25% agent + 50% differentagent + 80% (Comparative agent lime lime system) lime example (Examplea) (Example b) (Example c) (Example d) (Example e) Slag composition %CaO 90 53.8 65.4 75.8 75.8 90.1 T-Fe — 18.2 14.0 9.3 9.3 4.2 SiO₂ — 8.86.8 4.6 4.6 2.1 Al₂O₃ — 4.1 3.2 2.2 2.2 1.0 T-S — 1.8 1.4 1.0 1.2 0.5CaF₂ 5 1.8 1.4 1.1 1.2 0.6 CaCO₃ — 1.3 1.1 0.7 0.8 0.6 Ca(OH)₂ — 1.0 0.80.6 0.7 0.6 (Unit: mass %)

TABLE 18 Average desulfurizing agent composition used when bothrecovered desulfurizing agent and lime source Recovered desulfurizingExample agent Lime CaCO₃ Ca(OH)₂ a 100 — — — b 70 — 30 — c 60 — 20 20 d50 50 — — e 30 50 10 10 (Unit: mass %)

TABLE 19 Test conditions Item Contents Desulfurizing Mechanicallyagitating type method desulfurizing apparatus Treatment Hot metal ladlecontainer Throughput   140-160 t Hot metal  1300-1450° C. temperatureHot metal [S]  0.05-0.02%

TABLE 20 Amounts of recovered desulfurizing agent and lime source addedwhen 8.0 kg/T of CaO component is required in desulfurizing agentRecovered desulfurizing agent Added desulfurizing agent Total Cao Amountof Ca(OH)₂ amount component recovered Lime CaCO₃ Amount of of indesulfurizing Cao Amount of Amount of Ca(OH)₂ desulfurizingdesulfurizing agent added component lime added CaCo₃ added added agentagent wt % kg/T wt % wt % kg/T wt % kg/T wt % kg/T kg/T kg/T 100 11.4 70— — — — — — 11.4 8.0 20.0 40 — — — — — — 20.0 70 12.5 40 — — 30 5.4 — —17.9 60 9.5 40 — — 20 3.2 20 3.2 15.9 50 5.7 40 50 5.7 — — — — 11.4 303.2 40 50 5.3 10 1.1 10 1.1 10.6 (Unit: mass %)

TABLE 21 Average desulfurizing agent composition used when bothrecovered desulfurizing agent and C source are used C source AmountRecovered of C desulfurizing source Adding agent Type added methodExample 100 — 0 — a 95 Coke 5 Mixture b 95 Anthracite 5 Mixture c 80Coke 20 Mixture d 70 Coke 30 Separate addition (Unit: mass %)

TABLE 22 Test conditions Item Contents Desulfurizing Mechanicallyagitating method type desulfurizing apparatus Treatment Hot metal ladlecontainer Throughput   140-160 t Hot metal  1300-1450° C. temperatureHot metal [S]  0.05-0.02%

1. A method of manufacturing a hot metal desulfurizing agent, the method being comprising executing a treatment for creating a new surface in desulfurization slag resulting from a mechanical agitation type hot metal desulfurizing treatment.
 2. A method of manufacturing a hot metal desulfurizing agent for use in a mechanical agitation type hot metal desulfurizing treatment according to claim 1, the method being comprising executing a treatment for creating a new surface in desulfurization slag resulting from the mechanical agitation type hot metal desulfurizing treatment.
 3. A method according to claim 1, comprising a step of providing desulfurization slag resulting from the mechanical agitation type hot metal desulfurizing treatment and a step of executing the treatment for creating a new surface in the provided desulfurization slag.
 4. A method according to claim 1, wherein the step of executing the treatment for creating a new surface includes crushing a desulfurization slag grain and/or separating an aggregate of a plurality of desulfurization slag grains into desulfurization slag grains.
 5. A method according to claim 1, wherein the step of executing the treatment for creating a new surface includes crushing the desulfurization slag grain and/or separating the aggregate of a plurality of desulfurization slag grains into desulfurization slag grains by air-cooling the desulfurization slag and/or applying mechanical energy to the desulfurization slag.
 6. A method according to claim 5, wherein the desulfurization slag is cooled using one or two methods selected from the group consisting of air cooling and water cooling.
 7. A method according to claim 6, wherein the air cooling is carried out using one or two methods selected from the group consisting of natural air cooling and forced air cooling.
 8. A method according to claim 6, wherein the step of executing the treatment for creating a new surface by water cooling comprises a step of watering the desulfurization slag, and this watering step controls the amount of water provided so as to maintain the temperature of the desulfurization slag at 100° C. or higher at the end of the watering, so that only by means of cooling based on the watering, the aggregate of desulfurization slag can be separated into desulfurization slag grains and/or the desulfurization slag grains can be crushed.
 9. A method according to claim 1, wherein the step of executing the treatment for creating a new surface comprises a step of water-cooling the desulfurization slag and a step of drying a recovered desulfurizing agent resulting from the water cooling.
 10. A method according to claim 1, wherein the step of executing the treatment for creating a new surface comprises a step of cooling the desulfurization slag and a step of adjusting the grain sizes of the desulfurization slag and the recovered desulfurizing agent.
 11. A method according to claim 10, wherein the step of adjusting the grain size of the recovered desulfurizing agent using a sieve is executed at a temperature of 600° C. or higher.
 12. A method according to claim 1, wherein the step of executing the treatment for creating a new surface comprises a step of executing at least one or two treatments selected from the group consisting of a treatment for magnetically separating and removing metal from the desulfurization slag or the recovered desulfurizing agent, a treatment for removing large masses from the desulfurization slag or the recovered desulfurizing agent to set the grain size to 100 mm or smaller, and a treatment for setting the temperature of the desulfurization slag or the recovered desulfurizing agent to 200° C. or lower.
 13. A method according to claim 1, wherein the step of executing the treatment for creating a new surface comprises a step of setting the grain size of the desulfurization slag to 100 mm or smaller and the temperature thereof to 200° C. or lower. 14-32. canceled
 33. The method of manufacturing low-sulfur hot metal, the method being comprising desulfurizing hot metal by adding, to the hot metal, a hot metal desulfurizing agent comprising a recovered desulfurizing agent having a new surface created therein resulting from a mechanical agitation hot metal desulfurizing treatment.
 34. The method of manufacturing low-sulfur hot metal, the method being comprising desulfurizing hot metal using a mechanical agitation hot metal desulfurizing treatment by adding, to the hot metal, a hot metal desulfurizing agent comprising a recovered desulfurizing agent having a new surface created therein resulting from the mechanical agitation hot metal desulfurizing treatment.
 35. The method of manufacturing low-sulfur hot metal according to claim 33, the method being comprising desulfurizing the hot metal by adding, to the hot metal, the desulfurizing agent comprising the recovered desulfurizing agent having a new surface created therein resulting from the mechanical agitation hot metal desulfurizing treatment.
 36. The method of manufacturing low-sulfur hot metal according to claim 33, the method being comprising desulfurizing the hot metal by adding, to the hot metal, the desulfurizing agent comprising the recovered desulfurizing agent having a new surface created therein which results from the mechanical agitation hot metal desulfurizing treatment and in which part or all of an aggregate of the recovered desulfurizing agent is separated into pieces.
 37. The method according to claim 33, desulfurizing the hot metal by adding, to hot metal, the recovered desulfurizing agent having a new surface created therein resulting from the mechanical agitation type hot metal desulfurizing treatment and one or more sources selected from the group consisting of a lime source and a carbon source.
 38. The method according to claim 33, desulfurizing hot metal by mixing the recovered desulfurizing agent resulting from the mechanical agitation hot metal desulfurizing treatment with one or more sources selected from the group consisting of a lime source and a carbon source, and adding this mixture to hot metal.
 39. The method according to claim 33, desulfurizing the hot metal by separately adding, to the hot metal, the recovered desulfurizing agent resulting from the mechanical agitation hot metal desulfurizing treatment and one or more sources selected from the group consisting of a lime source and a carbon source.
 40. The method according to claim 37, wherein when the lime source is added, the mixing ratio is adjusted so as to obtain a predetermined amount of pure CaO component.
 41. The method according to claim 37, comprising a step of calculating a bulk density of the recovered desulfurizing agent resulting from the mechanical agitation hot metal desulfurizing treatment, a step of calculating an amount of pure CaO component in the recovered desulfurizing agent from the calculated bulk density, and a step of adjusting an addition ratio of the crushed recovered desulfurizing agent to the lime source on the basis of the calculated amount of pure CaO component in the recovered desulfurizing agent.
 42. The method according to claim 37, comprising a step of adjusting the grain size of the carbon source to 1 mm or smaller when the carbon source is added. 